Recently, temperatures and pressures in thermal power-plant boilers have been elevated remarkably. Operation at 566.degree. C. and 316 bar is planned for some plants, and future operating condition of up to 649.degree. C. and 352 bar is expected. Thus, the requirements for the plant materials have become more severer.
The heat-resistant materials used for thermal power plants are exposed to different environments depending on the portions at which the materials are employed. Materials having high corrosion resistance and strength at high temperatures, typified by austenite type materials, are used for portions exposed to a high atmospheric temperatures such as so-called "superheater pipes" and "reheater pipes", while martensite type materials containing 9 to 12% of Cr are used for portions where excellent steam oxidation resistance and thermal conductivity are required.
Recently, novel heat-resistant materials that contain W in order to improve the high temperature strength have been developed and put into practical application, and have made great contributions to the achievement of higher efficiency in power generation plants. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 63-89644, 61-231139 and 62-297435 describe ferritic heat-resistant steels capable of achieving much higher creep strength in comparison with Mo-addition type ferrite steels according to the prior art by using W as a solid solution strengthening element. Most of these materials have a structure of a tempered martensite single phase, and are expected to be used, as the materials of the next generation, in a high temperature/high pressure environments due to superiority of steam oxidation resistance in combination with high strength of the ferrite steel.
As higher temperature and higher pressure have been achieved in the thermal power plants, severe operating conditions have been imposed on those portions which have so far been exposed to relatively low temperatures and pressures, such as furnace wall pipes, economizers, steam generators, main steam pipes, and so forth. In consequence, the application of low Cr ferritic heat-resistant steels stipulated by the industrial standard, such as so-called 1.25 Cr steel and 2.25 Cr steel, has become gradually impossible.
To cope with such a trend, a large number of steels that have improved the high temperature strength by positive addition W or Mo, have been proposed for such low strength materials, too.
Japanese Unexamined Patent Publication (Kokai) Nos. 63-18038 and 4-2680040 and Japanese Examined Patent Publication (Kokoku) Nos. 6-2926 and 6-2927 propose 1% to 3% Cr-containing steels that contain W as a principal strengthening element and have improved high temperature strength. All of them have higher high-temperature strength than the conventional low Cr containing steels.
On the other hand, the ferritic heat-resistant materials utilize a property of the steel in that the phase transformation from the austenite single phase region to the ferrite+carbide precipitation phase, that occurs with cooling at the time of heat-treatment, exhibits a super-cooling phenomenon. These materials utilize also the high strength of the resulting martensite or bainite structure involving large quantities of transition, or its tempered structure. Therefore, when this structure receives the thermal history such that it is again heated back to the austenite single phase region, such as when it is affected by welding heat, the high density transition is again released. In consequence, the drop of strength occurs locally in the welding heat affected zone. Among the portions re-heated to a temperature higher than the ferrite-austenite transformation point, the portions heated to a temperature near the transformation point, such as 800 to 900.degree. C. in the case of 2.25% Cr steel, and again cooled within a short period of time, undergo again the martensite transformation or the bainite transformation and change to a fine grain structure before the austenite crystal grains grow sufficiently. Moreover, M.sub.23 C.sub.6 type carbide, that is the principal factor for improving the material strength by precipitation strengthening, does not undergo re-solid solution, but its constituent components are denatured or get coarsened. These mechanisms inviting the drop of the high temperature strength operate in complex ways and sometimes cause a locally softened zone. This softened zone generation phenomenon will be hereinafter referred to "HAZ softening" for convenience.
The inventors of the present invention have conducted intensive studies on the softened zone, and have found that the drop of the strength results mainly from the change of the constituent elements of the M.sub.23 C.sub.6 type carbide. As a result of further studies, the present inventors have found that large quantities of Mo or W, as the indispensable element for solid solution strengthening of the high strength martensite type heat-resistant steel, in particular, undergo solid solution into the constituent metal element M in M.sub.23 C.sub.6 while they are being affected by the welding heat, and precipitate on the crystal grain boundary of the structure which is converted to the fine grain structure. As a result, a Mo- or W-lean phase is generated in the proximity of the austenite grain boundary and results in the local drop of the creep strength.
Therefore, the drop of the creep strength due to the influence of welding heat is critical for the heat-resistant materials, and it is obvious that the prior art technology such as optimization of heat-treatment and a welding process cannot fundamentally solve this problem. Moreover, the application of counter-measure by converting again the weld portion to the complete austentie, that is believed to be the only solution, is obviously impossible in view of the construction process of the thermal power plants. It is also obvious that the "HAZ softening" phenomenon is quite unavoidable in the heat-resistant martensite steels or ferrite steels according to the prior art.
Notwithstanding the fact that the novel low Cr ferritic heat-resistant steel containing W or Mo has a high base metal strength, a drop in strength to a maximum of 30% occurs in the welding heat affected zone in comparison with the base metal. Therefore, this material has been regarded as the material having a small effect of improving locally the strength for the conventional materials. In Japanese Unexamined Patent Publication (Kokai) No. 8-134584, the present inventors have proposed a high strength ferritic heat-resistant steel excellent in HAZ softening resistance and a method of producing the steel. The steel according to this previous patent application contains, in terms of mass %, 0.01 to 0.30% of C, 0.02 to 0.80% of Si, 0.20 to 1.50% of Mn, 0.50 to less than 5.00% of Cr, 0.01 to 1.50% of Mo, 0.01 to 3.50% of W, 0.02 to 1.00% of V, 0.01 to 0.50% of Nb, 0.001 to 0.06% of N, at least one of 0.001 to 0.8% of Ti and 0.001 to 0.8% of Zr either alone or in combination, limits P, S and O to not greater than 0.030%, not greater than 0.010% and not greater than 0.020%, respectively, or contains at least one of 0.2 to 5.0% of Co and 0.2 to 5.0% of Ni, and the balance consisting of Fe and unavoidable impurities, wherein the value of (Ti % + Zr %) in metal components M of a M.sub.23 C.sub.6 type carbide existing in the steel is 5 to 65. The production method of a high strength ferritic heat-resistant steel excellent in HAZ softening resistance comprises adding Ti and Zr to the steel in the course of 10 minutes immediately before tapping so that the value (Ti % + Zr %) in the metal component M in the M.sub.23 C.sub.6 type carbide existing in the steel becomes 5 to 65, temporarily stopping cooling at a temperature within the range of 880 to 930.degree. C. after solid-solution heat-treatment, and holding the steel at the same temperature for 5 to 60 minutes.
As the demand for electric power has been increasing in recent years, however, not only the power industry but also business companies in different business fields have now been granted to carry on a power business so long as they have power generation and supply setups. Thus, the principle of competition has been introduced into the power supply business. As a large number of power generation setups have thus been built up, the price competition of power has been introduced into the power business companies, and the reduction of the cost of construction of the power generation plants has become all the more important. The improvement of the strength of the boiler materials will result in the reduction of the thickness of heat-exchangers, and so forth, and contributes to the reduction of the material cost. In working and assembly processes of the materials, a reduction of the number of process steps or shortening of the process has been desired. In the case of the ferritic heat-resistant steel used particularly for those portions of the setups which bear a relatively low pressure load, the materials which can omit the heat-treatment after welding (hereinafter referred to as "PWHT" (Post Weld Heat-Treatment), that would otherwise require a long time and a high expense, have been needed because the strength of the materials themselves is relatively low.
However, a higher strength of the material is contradictory to omission of the pre- and post-weld heat-treatment, and omission of the heat-treatment in joints made of a material having a high strength is extremely difficult to attain from the aspect of hardenability. Lowering of the strength of the HAZ results also in the promotion of HAZ softening resistance. For these reasons, it has been believed to be impossible in the past to accomplish a technology, for reducing the power plant construction cost, that can simultaneously satisfy the improvement of the material strength, the improvement of the HAZ softening resistance and omission of the PWHT.